The present invention relates, in general, to electronic components and circuits for selecting signals having specific frequencies and signals within frequency bands and, in particular, to a variable inductor and associated circuitry for frequency and frequency band selection.
Many mixed signal and radio frequency applications require variable reactive elements (e.g., inductors or capacitors) to achieve tuning, band switching, phase locked loop functions, etc. Such reactive elements are used in some type of circuit where the reactive element is resonated with another reactive element. The desired result is a resonant circuit that has a response that can be tuned from one frequency to another dynamically.
Typically, this is accomplished using a varactor to provide a variable capacitance that is resonated with a fixed inductor. One problem with using a varactor that is integrated in a typical silicon process is that the tuning range is limited, particularly for xe2x80x9cfreexe2x80x9d varactors that use the standard BiCMOS diodes or FETs as the variable capacitance element. The tuning range typically is less than 2:1.
Varactors implemented with special processing steps to optimize the tuning range and/or the quality factor Q will cause the overall processing cost and turn around time to increase. When a large change in frequency is desired for a varactor that does not have the desired tuning range, a bank of capacitors might be used that can be switched in and out of the circuit selectively to provide a range of discrete capacitance values. A varactor is placed in parallel with this xe2x80x9cdiscrete variablexe2x80x9d capacitor to provide a smooth variation of capacitance around the larger steps provided by the switched bank. This is shown in FIG. 1.
Referring to FIG. 1, a varactor 10 is connected in parallel with a resonating inductor 12. A bank of capacitors 14 and series connected switches 16 is connected in parallel with varactor 10 and resonating inductor 12. Operation of switches 16 is controlled by a controller 18 from which control signals are supplied to selectively open and close the switches and thereby change the tuning of the circuit as capacitors 14 are switched into or out of the circuit. Controller 18 receives input signals along input lines 20 that define which of the capacitors 14 are to be switched into the circuit or switched out of the circuit.
A major disadvantage of a circuit, such as the one illustrated in FIG. 1, is that the effective xe2x80x9csmoothxe2x80x9d tuning range of the combined discrete capacitor/varactor structure is inversely proportional to the amount of fixed capacitance in parallel with the varactor. Therefore, if a large discrete capacitor is placed in parallel with a comparatively small varactor, the overall smooth tuning range of the combined structure will be significantly reduced. This means that more bits of resolution are required from the xe2x80x9cdiscretely variablexe2x80x9d capacitor bank to compensate for the lost smooth variation provided by the varactor that is placed in parallel. This increased resolution takes up valuable chip area resulting in increased cost.
Another disadvantage is that the capacitance of the discrete capacitor bank might have a strong dependence on process variation, causing a large statistical variation in the capacitance.
Finally, the quality factor Q of capacitors with sufficient capacitance per unit area to be used in a practical manner in a capacitor bank is usually lower than desired. These capacitors are typically MOS devices with severe limitations on the quality factor Q due to series resistance through the device well.
To overcome the shortcomings of prior art integrated inductors and prior art inductor/varactor tuning circuits, a new and improved integrated inductor and a new and improved inductor/varactor tuning circuit are provided by the present invention.
It is an objective of the present invention to provide a new and improved variable integrated inductor.
It is another objective of the present invention to provide a new and improved inductor/varactor tuning circuit.
A variable integrated inductor, constructed in accordance with the present invention, includes an inductor, a single loop winding positioned in proximity to the inductor and having an open circuit, and means for selectively closing the open circuit of the single loop winding to magnetically couple the single loop winding to the inductor and decrease the magnetic field of the inductor. A switched inductor/varactor tuning circuit, constructed in accordance with the present invention, includes this variable integrated inductor connected with a varactor.
It is to be understood that the foregoing general description of the present invention and the following detailed description of the present invention are exemplary, but are not restrictive of the invention.